After a disastrous party the night before, Tim (Domhall Gleeson) is summoned to his father's (Bill Nighy's) study. According to his father, the men in Tim's family have the ability to travel to and from the past simply by going to a dark place, making his hands into fists, and wishing to return to a moment in his past. To Tim's considerable surprise, it actually works.
Technically, About Time isn't really perfect. The plot hole that Tim, in his 21 years on earth, hadn't ever balled up his fists in a dark place and wished he could have done something again, accidentally discovering by himself that he could time travel, is a little hard to swallow. And, at one point, near the end, the story seems to meander aimlessly for a few minutes. Frankly, I was more than willing to suspend my disbelief of those minor complaints for Ms. McAdams, (who, I'm a little concerned, may become type-casted as the female love interest in more time-travel movies, e.g., The Time Traveler's Wife), and the other really terrific components of the film.
Still, About Time is a lot of fun and, if you have a little time yourself, you could certainly find worse ways to spend 123 minutes.